Preventing the copying of a document that can be read by a human reader. An image is printed using clear toner or ink, creating a difference in reflected light and diffused light that can be discerned by a human reader by holding the paper at an angle, but can not be detected by a copier scanner which is restricted to reading at right angles to the page.
There has been a need for a printer that can print a page that can be read but not copied. One method, described in U.S. Pat. Nos. 4,210,346 and 5,695,220, is to use a particular white toner and a particular white paper that are designed to have different diffused light characteristics at different angles. Of course, this system requires special, matched paper and toner. What is needed is a printer which produces a non-copyable print on whatever paper happens to be in the machine at the time.